Struct darling::Error

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pub struct Error { /* private fields */ }

Expand description

An error encountered during attribute parsing.

Given that most errors darling encounters represent code bugs in dependent crates, the internal structure of the error is deliberately opaque.

Implementations

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impl Error

Error creation functions

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pub fn custom<T>(msg: T) -> Error where
T: Display,

Creates a new error with a custom message.

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pub fn duplicate_field(name: &str) -> Error

Creates a new error for a field that appears twice in the input.

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pub fn duplicate_field_path(path: &Path) -> Error

Creates a new error for a field that appears twice in the input. Helper to avoid repeating the syn::Path to String conversion.

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pub fn missing_field(name: &str) -> Error

Creates a new error for a non-optional field that does not appear in the input.

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pub fn unknown_field(name: &str) -> Error

Creates a new error for a field name that appears in the input but does not correspond to a known field.

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pub fn unknown_field_path(path: &Path) -> Error

Creates a new error for a field name that appears in the input but does not correspond to a known field. Helper to avoid repeating the syn::Path to String conversion.

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pub fn unknown_field_with_alts<'a, T, I>(field: &str, alternates: I) -> Error where
T: 'a + AsRef<str>,
I: IntoIterator<Item = &'a T>,

Creates a new error for a field name that appears in the input but does not correspond to a known attribute. The second argument is the list of known attributes; if a similar name is found that will be shown in the emitted error message.

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pub fn unsupported_shape(shape: &str) -> Error

Creates a new error for a struct or variant that does not adhere to the supported shape.

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pub fn unsupported_format(format: &str) -> Error

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pub fn unexpected_type(ty: &str) -> Error

Creates a new error for a field which has an unexpected literal type.

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pub fn unexpected_lit_type(lit: &Lit) -> Error

Creates a new error for a field which has an unexpected literal type. This will automatically extract the literal type name from the passed-in Lit and set the span to encompass only the literal value.

Usage

This is most frequently used in overrides of the FromMeta::from_value method.


use darling::{FromMeta, Error, Result};
use syn::{Lit, LitStr};

pub struct Foo(String);

impl FromMeta for Foo {
    fn from_value(value: &Lit) -> Result<Self> {
        if let Lit::Str(ref lit_str) = *value {
            Ok(Foo(lit_str.value()))
        } else {
            Err(Error::unexpected_lit_type(value))
        }
    }
}

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pub fn unknown_value(value: &str) -> Error

Creates a new error for a value which doesn’t match a set of expected literals.

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pub fn too_few_items(min: usize) -> Error

Creates a new error for a list which did not get enough items to proceed.

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pub fn too_many_items(max: usize) -> Error

Creates a new error when a list got more items than it supports. The max argument is the largest number of items the receiver could accept.

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pub fn multiple(errors: Vec<Error, Global>) -> Error

Bundle a set of multiple errors into a single Error instance.

Panics

This function will panic if errors.is_empty() == true.

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impl Error

Error instance methods

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pub fn has_span(&self) -> bool

Check if this error is associated with a span in the token stream.

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pub fn with_span<T>(self, node: &T) -> Error where
T: Spanned,

Tie a span to the error if none is already present. This is used in darling::FromMeta and other traits to attach errors to the most specific possible location in the input source code.

All darling-built impls, either from the crate or from the proc macro, will call this when appropriate during parsing, so it should not be necessary to call this unless you have overridden:

FromMeta::from_meta
FromMeta::from_nested_meta
FromMeta::from_value

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pub fn flatten(self) -> Error

Recursively converts a tree of errors to a flattened list.

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pub fn at<T>(self, location: T) -> Error where
T: Display,

Adds a location to the error, such as a field or variant. Locations must be added in reverse order of specificity.

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pub fn at_path(self, path: &Path) -> Error

Adds a location to the error, such as a field or variant. Locations must be added in reverse order of specificity. This is a helper function to avoid repeating path to string logic.

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pub fn len(&self) -> usize

Gets the number of individual errors in this error.

This function never returns 0, as it’s impossible to construct a multi-error from an empty Vec.

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pub fn write_errors(self) -> TokenStream

Write this error and any children as compile errors into a TokenStream to be returned by the proc-macro.

The behavior of this method will be slightly different if the diagnostics feature is enabled: In that case, the diagnostics will be emitted immediately by this call, and an empty TokenStream will be returned.

Return these tokens unmodified to avoid disturbing the attached span information.

Usage

// in your proc-macro function
let opts = match MyOptions::from_derive_input(&ast) {
    Ok(val) => val,
    Err(err) => {
        return err.write_errors();
    }
}